RI 2008-1E 79

BEAR LAKE FORMATION MICROPROBE DATA

by Cheryl Hartbauer1

Microprobe data were collected as part of a study to the more prevalent of the two. Anorthite content of the test the hypotheses that the composition of the Bear Lake plagioclase ranges from An44 to An69. In addition, Formation (fi g. 1) refl ects progressive erosion of Tertiary some quartz phenocrysts were identifi ed in the felsic and Mesozoic units of the Alaska Peninsula, and that volcanic fragments. diagenetic minerals present in the Bear Lake Formation Twenty-six fragments were analyzed refl ect conditions implied by thermal maturity indicators. from sample 04RR152b. The majority of these fragments Five samples were analyzed in order to characterize the have been altered to the point that their compositions composition of volcanic rock fragments, other frame- no longer resemble those of volcanic rocks. Ground- work clasts, and diagenetic minerals. These samples are mass has been replaced by clay, and in some fragments all sandstones from outcrops of the Bear Lake Formation plagioclase is albitized or replaced by calcite (fi g. 6). in the Port Moller area of the Alaska Peninsula (fi g. 2). Compositionally, a number of fragments still appear Major oxide compositions were measured using the volcanic, but the total alkali–silica diagram could not Cameca SX-50 electron microprobe at the Advanced be used to classify these fragments due to mobility of Instrumentation Laboratory at the University of Alaska Na and K. Instead, the data were compared to average Fairbanks. volcanic rock compositions with emphasis on the rela- Three samples were taken from measured strati- tively non-mobile elements Si and Ti. These fragments graphic sections and were analyzed using a 2-micron were found to be of intermediate and felsic composition, beam with 15 KeV and 10 nA. Pore-lining clay of a sodic ranging from basaltic to (fi g. 7). illite composition and pore-fi lling zeolites were identi- In general, microprobe analysis of these fi ve samples fi ed in sample BL2-176 (fi g. 3). Chemical composition from the Bear Lake Formation shows that volcanic and cursory XRD analysis suggest that these zeolites are fragments are slightly to severely altered, and of felsic heulandite. Other minerals identifi ed in these samples and . This characterization include plagioclase (An1–68), potassium feldspar, will aid in provenance identifi cation. Recognition of hornblende, glauconite, and smectite. Albite (An1–5) compositional variations and refi nement of provenance occurs as a pore-fi lling mineral and replaces plagioclase interpretation for the Bear Lake Formation will provide of higher anorthite content. Samples BL5-92 and BL2- a foundation for interpreting the tectonic dynamics of 176 contain both zeolite and albite. the Alaska Peninsula during the Miocene. The identifi ca- Analysis of two additional samples focused on clas- tion of pore-fi lling and replacement minerals, such as sifying volcanic rock fragments. A 20-micron beam was zeolite and albite, helps to characterize the diagenetic used with 15 KeV and 10 nA. Individual phenocrysts in mineralogy of the Bear Lake Formation. Considering each clast were analyzed and averaged with measure- this mineralogy in conjunction with existing thermal ments of the groundmass to approximate a whole-rock maturation data will aid in understanding the diagenetic composition. For fragments with phenocrysts smaller conditions that this formation has experienced in the than 20 microns, individual points that included a combi- Port Moller area. nation of phenocryst and groundmass were measured and averaged to approximate a whole-rock composition. REFERENCES Forty-six volcanic rock fragments were analyzed Reifenstuhl, R.R., Shafer, D.C., Ryherd, T.J., Brizzolara, from sample 04RR163b. Both felsic and intermediate D.W., and Blodgett, R.B., 2004, Summary of May volcanic fragments were identifi ed, with a notable ab- 25–June 4, 2004, fi eld notes and samples, Puale sence of mafi c fragments (fi g. 4). Classifi cation based Bay and Wide Bay areas, Alaska Peninsula: Alaska on comparison with average major-oxide compositions Division of Geological & Geophysical Surveys Raw of volcanic rock types agrees extremely well with Data File 2004-3, 16 p. classifi cation based on the total alkali–silica diagram. Streckeisen, A., 1976, To each plutonic rock its proper Phenocrysts identifi ed include pyroxene, hornblende, name: Earth Science Review, v. 12, p. 1–33. and plagioclase (fi g. 5). Both clinopyroxene and or- thopyroxene were identifi ed, with clinopyroxene being

1Univeristy of Alaska, Department of Geology & Geophysics, P.O. Box 757320, Fairbanks, Alaska 99775-7320; [email protected] 80 RI 2008-1E Bristol Bay–Alaska Peninsula region, overview of 2004–2007 geologic research

AGE ROCK UNIT

Alluvial and QUATERNARY Volcanic glacial deposits rocks and deposits PLIOCENE Milky River Fm.

Bear Lake Formation MIOCENE

Unga Fm.

OLIGOCENE Meshik Stepovak Fm. TERTIARY Volcanics EOCENE

Tolstoi Formation Figure 1. Tertiary stratigraphic column for the Alaska PALEOCENE Peninsula (modifi ed from Reifenstuhl and others, 2004).

Alaska

Sandy Lake

Port Moller Bear 04RR163b Lake BL5-92 BL3-205 BL2-176

Herendeen Bay Port Moller

04RR152b

20 km

Figure 2. Location of Bear Lake Formation outcrop samples used in microprobe analysis. BL prefi x indicates samples from measured stratigraphic sections. Bear Lake Formation microprobe data 81

120 μ m

Z

Z Z Z Figure 3. Photomicrograph of pore-lining illite (arrow) and pore-fi lling zeolite (Z) in sample BL2-176.

Classification of Volcanic Rock Fragments Sample 04RR163b

14 13

12 11 /trachydacite

10

O rhyolite

2 9 O+K 2 8 basaltic 7 tephrite/basanite trachyandesite wt % Na 6 trachy- 5

4 basalt andesite 3 pico- basalt 2 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79

wt % SiO2

Figure 4. Total alkali–silica plot for volcanic rock fragments analyzed in sample 04RR163b. 82 RI 2008-1E Bristol Bay–Alaska Peninsula region, overview of 2004–2007 geologic research

Figure 5. Photomicrograph of vol- canic rock fragment in sample 04RR163b with hornblende (Hbl), clinopyroxene (Cpx), 0.24 mm and plagioclase (Plag) (An56) phenocrysts.

0.24 mm 0.24 mm

Figure 6. Photomicrograph of volcanic rock fragment in sample 04RR152b with calcite-altered plagioclase (arrow). Shown in plane-polarized light (left) and with crossed polars (right). Bear Lake Formation microprobe data 83

Classification of Volcanic Rock Fragments Sample 04RR152b

1.6

1.4

1.2

1.0 2

0.8 rhyolite %TiO 0.6

0.4 basaltic andesite andesite dacite 0.2

0.0 50 55 60 65 70 75

% SiO2

rhyolite rhyodacite dacite andesite basaltic andesite average Classification boundaries taken from Streckeisen (1976) boundaries for sub-alkalic rocks.

Figure 7. SiO2 vs TiO2 plot for selected altered volcanic rock fragments analyzed in sample 04RR152b. Classifi cation boundar- ies are taken from Streckeisen (1976) boundaries for sub-alkalic rocks. The dashed curve is an average trend for 18,170 relatively unaltered igneous rocks.